1. Field of the Invention
This invention relates to authentication systems and, more particularly to systems for authenticating remotely generated optical control signals.
2. Description of the Prior Art
U.S. Pat. No. 5,519,389 was issued to Tomar Electronics, Inc. of Gilbert, Ariz. on May 21, 1996 and is entitled “Signal Synchronized Digital Frequency Discriminator.” The disclosure of the Tomar '389 patent is hereby incorporated by reference.
The Tomar '389 patent at FIGS. 11-13 discloses a directional optical signal detector 86 which includes a directional tubular optical input port 84 which transmits to photodetector 102 only incoming optical signals falling within a relatively narrow angular range of the aiming direction of input port 84 itself. The Tomar '389 patent also discloses an electronic circuit capable of discriminating in a noisy signal environment a pulsed strobe flash lamp emitter. Such emitters are typically coupled to emergency vehicles such as police cars or fire trucks or to transit vehicles such as buses to activate a traffic signal preemption system. Traffic preemption emitters typically operate at a fixed, relatively fast pulse repetition rate such as, for example, fourteen pulses per second.
The patented optical signal detector disclosed in the Tomar '389 patent and as generally illustrated in FIG. 1 of the present document is coupled in proximity to or collocated with a traffic controller and a signal head. When an emergency vehicle which has activated its strobe flash lamp emitter to generate traffic preemption signaling pulses moves within signal acquisition range of an optical signal detector as illustrated in FIG. 1, the optical signal detection circuitry disclosed in the Tomar '389 patent detects that optical control signal and generates a preemption output signal identified in FIG. 1 as a “valid optical control signal.” Transmitting that preemption signal to the FIG. 1 traffic controller initiates a controller cycle which after a defined time delay interval activates the green signal light on the signal head aimed at the emergency vehicle or transit vehicle allowing that vehicle to pass through the controlled intersection on a priority basis out of turn with the normal traffic controller cycle.
The Tomar '389 patent application was filed in 1992 and issued in 1996. The patented Tomar traffic preemption control system has experienced great commercial success over the years. Because compatible high pulse repetition rate preemption strobe emitters are not patented, third party vendors, particularly since the advent of on-line purchasing and distribution, have been able to sell such emitters on the open market potentially to purchasers other than governmental entities such as police departments, fire departments and transit authorities.
As a result of current security and safety concerns, governmental entities having legitimate access to traffic preemption systems have been searching for a means to render their optically actuated traffic preemption systems secure such that they cannot be activated by unauthorized persons or vehicles utilizing unauthorized preemption compatible emitters. The present authentication system invention has been developed to meet that need.